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The New Zealand Railways Magazine, Volume 2, Issue 6 (October 1, 1927)

Automatic Signalling. — Interlocking General

page 30

Automatic Signalling.
Interlocking General

Signalling might be defined as the method of governing train movements over certain areas of track, and interlocking as the detailed manner in which safety is assured. For a broad consideration of the subject it should be borne in mind that the signalling systems in use in New Zealand comprising Line Clear, Train Staff, Electric Tablet, Lock and Block, and “Automatic” operate to ensure safety and facilitate train movements between stations. Interlocking of points, signals, and signalling systems, is designed to prevent conflicting movements of vehicles and speed the handling of rolling stock at stations, junctions, sidings, goods yards, etc.

All-electric power interlocking machine, type “A.” The front cover plates have been removed, showing the levers (40 in number) and the connections to electric locking slides.

All-electric power interlocking machine, type “A.” The front cover plates have been removed, showing the levers (40 in number) and the connections to electric locking slides.

Though generally applied to the method of so governing the various movements of vehicles inside station or shunting yard limits that conflicting movements cannot occur nor signals show “clear” without their correct routes being set, interlocking has also a wider application. This relates to sections of railway and the co-ordination of safety appliances with systems of signalling.

Owing to the rapid advance of electric interlocking, the area and distance within which points and shunting movements can be controlled from an interlocking machine by one man with perfect knowledge of what is taking place, is ever widening. At no distant date sections of line, up to perhaps twenty miles, will be controlled, and sidings and wayside points actually operated, from a central interlocking station. This will be accomplished by electric control of distant motor-worked points, with visual indication of train movements enabling the signalman to watch and co-ordinate main line and shunting requirements in his section.

Mechanical and Electric Interlocking.

No wide difference in principle exists between the mechanical and electric systems of yard control. Each has its proper place in view of the facilities desired. In actual practice the essential difference in system consists of the “track circuiting” of the main running lines in electric “interlockings” preventing a train receiving a clear signal into an occupied track. In many instances a combination of mechanical and electric operation provides the most flexible method of working.

The main factors suggesting an electric installation are:—


An available power supply.


To obtain concentration of control. (One electric machine can operate the longest yard whereas mechanical operation of points is limited to approximately 250 yards.)


Small stations, where its installation can enable the clerical staff to operate the signalling, as no heavy manual work is then required.


Very large installations, where the reduction in the number of levers required for electric interlocking permits one man to operate the yard.


The advantage of track circuiting of main running lines to provide protection for standing trains, thus supplementing the human factor.


The assistance to enginemen in speed control. Electric or partial electric interlocking facilitates the use of Three Position (speed) signalling.


The benefit of an illuminated diagram to provide visual indication of yard conditions to the signalman.

page 31

With mechanical interlocking alone, the onus of ascertaining that a route is clear before signalling a train in rests with the signalman. With combined electric and mechanical interlocking, in addition to the usual mechanical safeguards, the track circuiting of the main running lines governs the signals giving access to those lines, so that when a track is occupied its signal lever (if a mechanical signal) is locked in the normal or danger position. With colour-light or power-worked signals, an occupied track automatically keeps its entering signal at danger, whatever the position of the lever.

Electric interlocking considerably modifies but does not eliminate the mechanical locking governing the various combinations of levers which may be pulled over together. It supplements the mechanical locking by providing additional locks on the levers and a further check on the signal indications where such is made necessary through the track being occupied, the permanent way damaged, or faulty adjustment, etc.

Interlocking A Station.

Having decided, on the grounds indicated above, whether a mechanical, all electric, or partial electric system is most advisable at any new installation to be undertaken, the interlocking expert proceeds as follows:

The various movements and combinations of movements required for the most efficient working of the yard are determined by ascertaining the requirements of the Traffic and Locomotive branches, together with particulars of train lengths to be handled and the density of traffic to be provided for. An eye must also be kept on probable extension of the interlocked area and projected alterations in the near future. This information is co-ordinated with certain practice derived from the experience of other countries, modified as required to suit New Zealand conditions.

The required signals are then placed on the yard diagram, and their levers in the interlocking machine allotted. The points and trap points in the interlocked area, with the facing point locks (if mechanical installation) are also given lever numbers, thus deciding the size of the machine required.

The locking plate is then designed, in terms of lever numbers showing all possible combinations of levers that can be pulled together with safety and yet fulfil the requirements stated above.

A Mechanical Installation.

A central site, if possible overlooking the whole interlocked area, is selected for the signal cabin. Within this area, which generally comprises the running roads and those points giving access to the running roads from sidings, goods yard, engine sheds, etc., all crossovers, points, and facing point locks are connected by rodding to their levers in the signal cabin.

Signals and shunting signals are also connected to their respective cabin levers by a stranded steel wire. Trap points may have
Interlocking Machine, Electric Pneumatic Signalling, Dunedin.

Interlocking Machine, Electric Pneumatic Signalling, Dunedin.

page 32 separate levers or be worked in conjunction with nearby points. Trap points are always indicated on the ground by square targets which turn with the points and not separately connected to the interlocking machine. This machine where possible, is so placed in the cabin that the signalman faces the yard as he works, having No. 1 lever on his left.

The main line signal levers are placed at the ends of the machine, those on the signalman's left controlling the trains moving from left to right.

Points, points lock, and shunting signal levers are so grouped in the centre of the machine as to eliminate unnecessary walking for the operator. The machine (or frame) consists of these levers, their undergear to connect with points and signals, and the locking plate.

Though the lever handles in the top story-all gaily arrayed in paint and polished metal-make the braver show, the most important part of the work is performed on the ground floor.

In the lower story are the cranks and wires connecting the machine to the outside system. Also close under the upper floor is placed the locking plate, which is the brain of the machine.-(To be continued.)

Department's Helpful Attitude.

Recently, the Thames Chamber of Commerce was represented on a deputation of Associated Chambers of Commerce, which requested the Department to arrange for a special carriage of mails by the early morning steam rail-car (says the Thames “Star”). This had been acceded to. It was, therefore, considered that due reciprocity was called for on the part of those who had benefited by this further evidence of the Department's willingness to go out of its way, on occasion, to foster the interests of the business community. Commenting on this aspect of the position, Mr. Wellsted, Business Agent of the Department, pointed out that disbursements in railway men's wages at Thames amounted to £5,000 annually, by far the major part of which was spent locally. This, the Department would say was a good and sufficient reason, apart from any other consideration, for a further measure of patronage of the railway goods service by Thames business people.

Addington North Box(Mechanical). Lock and Block Instruments and Interlocking Machine.

Addington North Box(Mechanical).
Lock and Block Instruments and Interlocking Machine.